Nerve cell assemblies in the circumscribed regions of the brain are pathologically, e.g. excessively synchronously, active in patients with neurological or psychiatric diseases such as Parkinson's disease, essential tremor, epilepsy, functional disturbances after a stroke, dystonia or obsessive compulsive disorders. In this case, a large number of neurons synchronously form action potentials, i.e. the participating neurons fire excessively synchronously. In a healthy person, in contrast, the neurons fire with a different quality, i.e. in an uncorrelated manner, in these brain sectors.
In Parkinson's disease, the pathologically synchronous activity changes the neuronal activity in other brain regions, e.g. in areas of the cerebral cortex such as the primary motor cortex. In this respect, the pathologically synchronous activity in the region of the thalamus and of the basal ganglia, for example, imposes its rhythm on the cerebral cortex areas such that ultimately the muscles controlled by these areas develop pathological activity, e.g. a rhythmic trembling (tremor).
Deep brain stimulation is used to treat Parkinson's patients who cannot be sufficiently treated by medication. In this process, deep electrodes are implanted in specific areas of the brain, e.g. in the subthalamic nucleus. An electrical stimulation is carried out via the deep electrodes to relieve the symptoms. With the standard high-frequency stimulation for treating Parkinson's disease, a so-called high-frequency permanent stimulation is carried out at frequencies of more than 100 Hz. This type of treatment has no long-lasting therapeutic effects (cf. P. Temperli, J. Ghika, J.-G. Villemure, P. Burkhard, J. Bogousslaysky, and F. Vingerhoets: How do Parkinsonian signs return after discontinuation of subthalamic DBS? Neurology 60, 78 (2003)). “Coordinated reset stimulation” (CR stimulation), that can additionally have long-lasting therapeutic effects, manages with less stimulus current (P. A. Tass, L. Qin, C. Hauptmann, S. Doveros, E. Bezard, T. Boraud, W. G. Meissner: Coordinated reset neuromodulation has sustained after-effects in Parkinsonian monkeys. Annals of Neurology 72, 816-820 (2012); I. Adamchic, C. Hauptmann, U. B. Barnikol, N. Pawelcyk, O. V. Popovych, T. Barnikol, A. Silchenko, J. Volkmann, G. Deuschl, W. Meissner, M. Maarouf, V. Sturm, H.-J. Freund, P. A. Tass: Coordinated Reset Has Lasting Aftereffects in Patients with Parkinson's Disease. Movement Disorders (published online, 2014)).
With other diseases, e.g. epilepsy that cannot be sufficiently treated with medication, different electrodes, e.g. epicortical or epidural electrodes, are also implanted in addition to deep electrodes. With further diseases, e.g. chronic pain syndromes, it is customary to stimulate the spinal cord not only by means of deep electrodes in the brain, but also by means of e.g. epidural electrodes. In contrast to CR stimulation, most other types of stimulation have no long-lasting therapeutic effects.
Therapeutic effects can also be achieved by direct stimulation of the brain tissue or spinal cord by light, e.g. via implanted light-guides. Different spatiotemporal stimulation patters such as CR stimulation can also be used in this respect.
Limitations can arise in the conventional treatment of brain diseases and spinal cord diseases in which electrodes or light-guides or comparable stimulation units are implanted in the brain and/or spinal cord of the patient to achieve therapeutic effects by an electrical and/or optical stimulation of the brain tissue or of the spinal cord. Such stimulation treatments can cause side effects, e.g. due to the unwanted stimulation of adjacent structures as a result of the propagation of stimulus currents or due to the simultaneous stimulation, which is difficult to avoid for anatomical reasons, of e.g. fibrous webs and/or fibers that run in the vicinity of the target region or even through the target region. Such situations result e.g. due to the characteristic close anatomical proximity of the target point targeted in the electrode implantation and of other anatomic structures (whose stimulation results in side effects), due to special individual anatomical conditions (e.g. in the sense of the location of blood vessels that have to be spared on the implantation of the electrodes) or also due to suboptimal or even erroneous electrode implantation.
For example, side effects can occur due to unfavorably positioned electrodes that only disappear when the stimulation level is reduced so much that the desired effects do not occur to a sufficient degree or do not even occur at all. Analogously, an insufficient stimulation effect cannot be compensated by an increase in the stimulation intensity by any desired amount since hereby side effects typically occur or even damage to the tissue is caused. Where necessary, badly positioned electrodes or other stimulation units have to be re-implanted to position them such that the treatment is efficient. A repeat implantation is always associated with a risk, e.g. due to an injury to vessels or due to an infection.